There are various free disassemblers available which can turn 8052 binary files back in to assembly code. Use the Search facility on this page, with keyword 'disassembler' to find references.

Haven't seen anything which attempts to go all the way back to C. This topic (and some of the difficulties) are discussed at
http://compilers.iecc.com/comparch/.../90-09-070

Bear in mind that you should not reverse engineer or reuse code which somebody else has written unless you ask for permission first.